1. Field of the Invention
The present invention relates to a manufacturing method for a porous microneedle array and a corresponding porous microneedle array and a corresponding substrate composite.
2. Description of Related Art
Although applicable to any micromechanical components, the present invention and the related background on which it is based will be explained with regard to micromechanical components in silicon technology.
Porous microneedle arrays, which are made for example of porous silicon, are used in the area of transdermal drug delivery as an extension of medicinal patches, as carriers of a vaccine or also for removing body fluid (so-called transdermal fluid) for the diagnosis and analysis of body parameters (such as glucose, lactate, alcohol, etc.).
Medicinal patches (transdermal patches) for small molecules (such as nicotine) are widely known. The range of utilization is normally limited to small molecules that are easily able to pass through the skin. In order to expand the range of use of such transdermal applications to other active agents, so-called chemical enhancers or various physical methods (ultrasound, heat pulses) are used, which help to defeat the protective layer of the skin even for larger agent molecules.
Another method for doing this is mechanical penetration of the outer layers of the skin (stratum corneum) by fine, porous microneedles combined with the administration of an active agent, preferably using a patch containing an active agent, into which the microneedles may already be integrated, or using a metering device which enables targeted dispensation (bolus, pause, increase, etc.) of active agents.
Published German patent application document DE 10 2006 028 781 A1 discloses a method for manufacturing porous microneedles, arranged in an array on a silicon substrate, for transdermal administration of medications. The method includes forming a microneedle array having a plurality of microneedles on the face of a semiconductor substrate, which rise above a carrier zone of the semiconductor substrate, as well as partial porosification of the semiconductor substrate to form porous microneedles, the porosification starting from the face of the semiconductor substrate and a porous reservoir being formed in the interior of the structures.
Another method for manufacturing a porous microneedle array is known from published Chinese patent application document CN 100998901A.
The previously known approaches provide for manufacturing porous microneedle arrays on the wafer face using known plasma etching techniques. After the microneedle arrays have been manufactured, the wafer is anodized electrochemically in aqueous hydrofluoric acid by applying electrical potentials, whereby the material silicon is transformed into porous silicon starting from the wafer face having the microneedle array. This porosification cannot proceed extensively throughout the entire wafer, however, because the first pore that reaches the back of the wafer forms an electrical short circuit or bypass around the wafer, so that further anodization is slowed or comes to a stop after a certain time. Moreover, it is very time-consuming and uneconomical, or even impossible, to anodize through an entire wafer thickness, or even only several hundred μm of a wafer thickness.
For that reason, use is often made in the related art of so-called ablating processes, in which the wafer is thinned to a lesser wafer thickness prior to anodizing, for example by mechanical grinding or chemical wet etching or plasma etching in the gaseous phase. That shortens the time needed for through-porosification of the wafer thanks to its reduced thickness, although at the cost of an increased risk of breaking during this process or other subsequent processes.
Furthermore, it remains impossible, even with a thinned wafer, to anodize the latter over its complete extent through the entire remaining wafer thickness, due to the previously mentioned electrical bypass problem through the first pores that extend all the way through the wafer.
For this reason, for example, metal layers and/or metal foils are applied to the back of the wafer, which may be removed again after the anodizing is completed. It is also possible to anodize the wafer only partially, either until the first pores reach the back, but most of the pores have not yet gotten that far, or right from the outset to anodize only to a certain preselected depth in the silicon bulk material. In both cases, material is next ablated mechanically or chemically from the back of the wafer until a completely porous structure that extends through the remainder of the wafer is achieved. Another positive effect achieved with the ablation is that the remaining silicon is significantly thinner than that of the initial wafer, which makes it easier for active agents to pass through into the skin or for transdermal fluid to pass out of the skin, in one direction and in the other direction.
With these grinding or etching procedures, however, there is great expense, mechanical stress on the wafer, a risk of breaking, and a need to protect the microneedle array present on the face from mechanical damage. Since silicon is a brittle and breakable material, there is a risk of damage when handling the front of the wafer, as is necessary for example when grinding the back of the wafer. However, temporary protective measures during the process mean an additional, sometimes substantial, processing expense.